Rainwater-induced migration of potentially toxic elements from a Zn–Pb slag dump in Ruda Śląska in light of mineralogical, geochemical and geophysical investigations

Abstract

The Upper Silesia region of Poland is known for its concentration of heavy industry, including mining and smelting. Slag dumps scattered across the region are the sources of pollutants such as Zn, Mn, As, Cd and Pb. At the slag dump in Ruda Śląska, it is possible to distinguish three types of slag: (i) slag associated with the muffle/lining material, composed of olivine, pyroxene, iron oxide, feldspar and glass; (ii) glassy slag composed of glass and single crystals of melilite and wollastonite; and (iii) slag composed of aggregates of melilite, wollastonite and anorthite. These concentrate Potentially Toxic Elements (PTEs), reaching up to 6130 mg/kg of As, 36300 mg/kg of Mn, 21700 mg/kg of Pb, 53600 mg/kg of Zn and 105 mg/kg of Cd. Rainfall-induced weathering causes PTE mobilisation to secondary phases, mainly carbonates, sulphates, oxides and hydroxides. Leaching tests based on the Synthetic Precipitation Leaching Procedure (SPLP) prove the release of PTEs from slags and secondary phases, which was further confirmed by significant PTE concentrations in a water sample from effluent next to the dump: 2710 μg/l Mn, 163 μg/l Zn, 52.5 μg/l Pb, 20.1 μg/l As, 0.98 μg/l Cd. This study is the first attempt to apply a combination of electromagnetic (EM) measurements and mineralogical investigations to pyrometallurgical Zn–Pb slags, and to precisely describe links between the obtained conductivity and in-phase results and phases building soils and sediments of the studied area. Contamination mapping, using combined scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD) and EM methods, allowed to distinguish the main migration path of PTEs from the slag dump. This path is related to the effluent in which the presence of sulphides, oxides and hydroxides concentrating Cd, Zn, and Pb was determined to cause both high conductivity (>300 mS/m) and in-phase values (>6 ppt). On the other hand, soils from other areas outside the dump are not polluted by this migration path. Application of EM mapping confirmed its usefulness as a tool for tracking environmental contamination. For successful interpretation, however, it must always be used in combination with mineralogical/petrological methods.